Exchange will only allow one instance of any email address, so it can't be done in the way
you ask.

Why do you need multiple contacts pointing to one address? If it's to redirect multiple SMTP addresses to one external address, just create the one contact with multiple addresses. If you want multiple entries in the address book that will divert externally, you could create mailboxes set to forward to the contact, or distribution groups that the contact is the only member of.

Which version of Exchange are you using? I've tried this on Exchange 2007 and 2013 (so 2010 will be the same), and Exchange only allows an address to be used for the ExternalEmailAddress attribute for a single contact.

The proxy address "SMTP:testaddress@testdomain.com" is already being used by "domainname.local/Users/testcontact1". Please choose
another proxy address.

So I believe that this can't be done. To achieve having multiple recipients in your address book that will send to the same address, I'd remove your existing contacts and replace them with a single contact, then for each recipient you want visible in the address book, create a distribution group that this contact is the only member of.

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In both of my comments, I suggested making the contact a member of multiple distribution groups in order to achieve what the requester wanted. The requester has then said that they've used exactly this method in their solution, but said that my reasons are not applicable!

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